I've been using Chladni patterns on my archtops right from the first one. I wrote it up in my three part plate tuning series in 'American Lutherie' back in '91-92: it should be in the GAL 'Red Book #3? Anyway, I've learned a few things since then which, although they don't change much of what's in the articles, are useful refinements.

As with violins, it's easier to check the 'free' plate modes on archtops by putting the plate on pads with the inside up, like a dish. The glitter runs down toward the center, but tends to stop on node lines. You sprinkle it up slope from where you expect the pattern to be, and it slithers down when you hit the frequency.

On guitars the 'X' mode doesn't seem to be very important; you're mostly working on the 'O' mode. I try to get those to have good shapes, be clearly defined in frequency, and I like to match the top and back 'O' mode frequencies if possible. I generally tune the plates before I cut in the holes or add in the bars. Usually what happens is that you can get decent looking modes at that point, but they get messed up when you cut the holes. Putting in the bars and tuning things gets the top 'O' mode back in shape and frequency (if you ate all of your vegetables). What's supposed to happen than is that the main top and back 'ring' modes come in vey close to the same pitch on the assembled box. The combination of the bridge mass and the down force of the strings drops the top mode pitch so that it's a semitone or a whole tone lower than the back. Sometimes it even works!

I'll see if I can dig out any pictures of this stuff, and I can also scan in the data sheets I keep on this for the guitars I make. I'm going to be pretty busy with a family wedding the next few days, but I'll try to keep track of the thread and give some input when I'm able.

(and please excuse my link into that other forum - i don't know a better way to refer to these).

Thanks Beate, I’ve found resonances near those frequency’s and ones used by flat tops.The using tea leaves the patterns are not as clear and more grouped in the center.The back showed the fewest resonances. I can still remove 1.5mm on the back most places.

Here is couple mode test. On a cherry top and back.These plates were molded in a vacuum bag.Two 1mm layers of cherry with a Flax fiber cloth/epoxy core. Total thickness is 2.5mm.The inner layer of cherry was mostly scraped off, on the top, as these were prototype testing samples.It is quite flexible and I think stiff as well.the plates are 1/4" wider than needed.

[quote="Eldon Howe"]Here is couple mode test. On a cherry top and back.These plates were molded in a vacuum bag.Two 1mm layers of cherry with a Flax fiber cloth/epoxy core. Total thickness is 2.5mm.The inner layer of cherry was mostly scraped off, on the top, as these were prototype testing samples.It is quite flexible and I think stiff as well.the plates are 1/4" wider than needed.

Well, I finally got around to scanning in some stuff on the most recent guitar. There are a couple of pictures of the tailpiece in another thread: here's the assembled box with the binding on it, which might give you more to go on. The top is, as far as I can tell, some old European spruce that I got from Carleen Hutchin's stash, and she got from the stash of a deceased maker in about 1960. The back is Broadleaf maple that I've had for at least 20 years.

The intent with this one was to make a passable acoustic copy of an original Lloyd Loar L-5, but not a physical copy. I used a slightly modified version of the Gibson lengthwise arch, and cycloid cross arches. The top graduations were loosely based on the original, which had been read off with a Hacklinger gauge, and the gist communicated to me. The outline is my own, but designed to fit in a standard L-5 case: never build one you can't get a case for. Although we have not had the chance as yet to try it out alongside the original, the owner is quite pleased with it, and I've been getting good reviews from other folks who have heard it.

Here are the final mode shapes and frequencies. Inexplicably I forgot to note down the the heights of the braces at the center and ends. I can only say they are more or less 'normal'. The graduation patterns are on another sheet: The top has an area 5mm thick at the bridge, slightly egg shaped, covering approximately the area beteween the holes. The edge is 3.5mm thick roughly from the wide part of the lower bout up around the top edge to about 50-60mm in from the edge. The lower edge at the block is 4.5m thick. The original was thinner outside of the f-holes: I was told about 3mm, but I elected to leave this one thicker as the mode shapes and frequencies were good. The back is a uniform 4.5mm thick over all.

The patterns on the right of the second sheet are the modes of the box as assembled, but without the binding and before the edge contours were totally finished. The surprise there was that the cross dipole mode actually came in at a lower pitch than the monopole: I've never seen that before. You might attribute that to the mass of the 'parallel' braces on the top, except that the back did the same thing with no bracing. It must, therefore, have to do with the arch shape.

In mode tuning on arch tops I have two goals. One is to get as many well formed modes as I can. I feel that one of the objectives of Chladni tuning is to make a plate that will work well in the higher frequency range, where it is hard to isolate things on the finished guitar. The 'free' plate mode shapes seem to me to be a sort of indicator of how smoothly the mass and stiffness are distributed, which seems a reasonable way to facilitate 'good' high frequency response.

The second objective, empirically derived, is to match the frequencies of the 'ring' modes in the free top and back. This tends to produce 'main top' and 'main back' modes at similar pitches on the assembled box, as you see here. Once the guitar is strung up the combination of bridge mass and down pressure drops the pitch of the 'main top' mode enough to avoid 'wolf' issues while still allowing for good coupling with the back, in most cases. On this instrument the 'main top' mode dropped to 210Hz when it was strung up, while the 'main back' mode rose to 264Hz. It's not unusual for the back mode to rise: tension along the body from the neck does it, but this one rose more than most. OTOH, in that repect it seems to have mimicked it's model: sometimes you get the bear, and sometimes it gets you.

This was a 'different' guitar for me: my first one with 'parallel' bracing, and a 'fuller' lengthwise arch than I normally use. I have also been using 'reverse graduations' of late: with the top a unifiorm thickness except for a slightly thinner area in the center. Those guitars have been pretty good, but this one is better. Since I changed several variables here it's hard to attribute the difference with any confidence. If anybody wants to see what those plates have been doing I could post some of that. Maybe even in less than a couple of months!

Now that I've gotten started I thought I'd upload something for comparison. This is data on my 'Winter' archtop, which I built as a showpiece back in '04. It's the same size and shape, but with a 'flatter' arch (same height but different contours), X bracing, different graduations, and a cutaway, of course. I'm probably not the best person to ask about the sound, but if I were pressed I'd say that #84 has a more 'modern' sound, while the newer #133 sounds more like a '30s instrument (which was the point!).

The materials on this one are European maple and spruce for the box, with American maple for the neck, and ebony trim.

Here is the data on the plates as they were just before I assembled the box. It's interesting to note that on 'winter' the back is graduated while the top is uniform thickness, while #133 is the reverse, but the modes are pretty similar. Also the plate weights are quite similar.

Here are spectrum charts of the two, along with the assembled mode data. A0 is the main air resonant frequency: it's lower on #133 due to the much smaller sound holes. This also makes the mode weaker as compared with the 'main top' mode. The tailpiece swings up and down, and can couple with the top, altering the sound if the resonance is close to a top or air resonance, so it's useful to look at that. That's what is causing the dip in the 'main air' peak on #84, and lowering it's amplitude. Note, though, that the area under the curve; the 'total available horsepower', could be greater on #84. You can barely see the tailpiece peak in the spectrum of #133. 'C-1' is the 'first corpus mode'; the entire guitar vibrating like a xylophone bar. If this is high enough in pitch it can couple with the main air resonance, again, causing a split peak and altering the tone. Here, as is common, it's too low in pitch. I would have expected the cross dipole mode on the top of #133 to be lower in pitch than that of #84 due to the difference in the top bracing, but that seems to have been overruled by the difference in the arch shape.

It's interesting to see how different the sppectra are on the two in the 'main top/main back' frequency range, given that the pitches of the modes are so similar. This is a complex interaction, since the top and back can couple both through air pressure changes in the box and also through the sides. The two sorts of coupling are out of phase, and that may be what's causing the dip in the tap spectrum at 235Hz on #84, and 241Hz on #133. Both of the output peaks are more or less cut off near the 'main back' resonant frequency, as the back steals energy from the top and doesn't radiate as effectively.

So there you go, boys and girls: something to puzzle over. Have fun. I hope I caught all the typos.

Thanks Alan, I did purchase this book.plate tuning series in 'American Lutherie' back in '91-92: it should be in the GAL 'Red Book #3So, my test plates came in low, so I’m adding the braces a little thick to see if this will help.

Alan, I'm puzzling over the vast differences between your FFT charts and mine. This is a 17" archtop with a douglas fir top and hard maple back and sides. I just muted the strings and tapped on the bridge to get this spectrum. I would have expected the results to be different than yours, but with more of a family resemblance. I'm guessing 136hz is the air resonance and the twin peaks around 222-240hz are the combined main top and back modes, but where would the 113hz peak come from? I'm new at this stuff, so maybe I'm doing it wrong. Any thoughts?

It's more likely that the tailpiece resonance is actually around 115 Hz or so, in the dip between the two peaks. The tailpiece is stealing energy from the top and it's not nearly as good at producing sound, even though it may be vibrating at a high amplitude. Try adding some weight to the tailpiece to drop it's resonant pitch, and see what happens. Poster adhesive is good for this. If you're using an adjustable tailgut you can make changes by moving the tailpiece closer to or further from the bridge. Jim D'Aquisto said that he could alter the sound of an archtop quite a lot by changing the tailpiece.

I used a very light tailpiece on my first archtop Classical guitar. After some testing I decided to add some weight to get it's pitch down to match the 'main air' frequency, so I stuck on a piece of metal with double-stick tape. I took it by to show a friend. He wasn't in, but his shop buddy was, and started to play it. My friend walked in, and after listening for a while he asked about the piece of metal. I reached over and pulled it off, and he exclaimed: "Who shut off the speaker?"Archtops give you a lot of dials to twist.

Alan Carruth wrote:I'm confused. Did you try both adding mass and damping the tailpiece with your hand? They're different.

Yes sir. These graphs show the tap response with (1) free tailpiece, and (2) 30g weight taped to the tailpiece. No hand damping in either, except for the strings. As I said, the only meaningful difference seems to be the loss of the first peak at around 113hz.